It's a number that is supposed to represent a player ID that players cannot have. This is why it's used in callbacks such as OnPlayerDeath, you know that there is no killer if the killerid is INVALID_PLAYER_ID because the SA:MP server is designed to do that so it's predictable. It's done like that so you know there's no killer associated with the event and it wouldn't get confused with all valid playerid's.

The meaning of a check like that can change from function to function.